Technical Background
Thiol- or selenol-containing compounds, e.g., cysteine, cysteamine, glutathione, selenocysteine, selenocysteamine, and the WR compounds, are known protective and preventive agents. Potential protective or preventive uses of such agents are widespread, as in reducing the unwanted side effects of chemo- or radiotherapy of cancer, improving cardiovascular function, preventing mutagenesis, preventing the initiation and/or progression of cancer, reducing toxic consequences of planned or unplanned radiation or chemical exposures, slowing the aging process, and preventing cataract formation. New evidence also links these compounds to altered gene expression and enhanced cellular repair processes.
The activity of these thiol- or selenol-containing compounds is mainly due to the sulfur or selenium atom participating in nucleophilic attack on toxic electrophiles, scavenging free radicals, effecting repair of damaged targets through hydrogen atom donation, altering the redox status of the cell, or affecting gene transcription or protein function.
For example, the reduced form of glutathione (Glu-Cys-Gly), a naturally occurring tripeptide with a free sulfhydryl group (SH), serves as a sulfhydryl buffer that maintains the cysteine residues of hemoglobin and other proteins in a reduced state. Glutathione also plays a key role in detoxifying the body by reacting with both endogenous and exogenous compounds, such as hydrogen peroxide and other peroxides.
Evidence suggests that glutathione is useful at protecting the body from the harmful side effects of radiation and chemotherapy that often accompany cancer treatment. Cyclophosphamide (CTX), for example, is a widely used antitumor agent whose clinical utility is limited by its bladder toxicity. During CTX metabolism in the body, a compound, acrolein, is released. Acrolein is thought to be responsible for the urotoxicity of CTX. Glutathione has been implicated in CTX detoxification by conjugating to acrolein.
It has been of significant interest in the art, therefore, to increase glutathione synthesis especially during periods of toxic insults. L-cysteine, a reactant in normal glutathione biosynthesis, is known to increase the synthesis of endogenous glutathione. To date, a significant challenge in the art has been to provide L-cysteine to cells at sufficiently high levels to drive glutathione biosynthesis. As disclosed, for example, in U.S. Pat. No. 4,868,114 to Nagasawa et al., prodrugs of L-cysteine (i.e., chemical compounds converted to L-cysteine in the cell), such as RibCys, can be used by the cell to drive glutathione biosynthesis shown below. 
These prodrugs have been shown to offer good protection against a variety of toxic insults. However, the initial prodrugs are highly water soluble and are rapidly excreted by the body.
WR compounds are also of significant interest in the art. Over 4400 WR compounds were prepared and tested at the Walter Reed Army Hospital after World War II in an effort to develop radioprotective compounds that might be employed by military personnel during a nuclear encounter. The single agent with the greatest potential that arose from that extensive effort was WR-2721, which is converted to WR-1065 by enzymatic cleavage. These compounds have several shortcomings, however, including that they possess noteworthy toxicity and little oral activity, greatly reducing their clinical utility.
Finally, selenocysteine are of significant interest in the art for their antioxidant and anticancer properties. In fact, selenium has received significant attention for its ability to inhibit or delay the onset of AIDS caused by HIV infection. Selenium is also a cofactor of glutathione peroxidase, an enzyme which has been implicated in many detoxifying processes.
Selenium is an essential mineral that is critical to the normal functioning of many species, including humans. It also has demonstrated activity as a cancer chemopreventive agent. Selenium-containing compounds appear to have especially high preventive activity against the initiation phase of colorectal cancer, although its chemoprotective ability has been extended to cancers in many organs, caused by a variety of carcinogens.
To achieve this chemopreventive activity, levels of selenium at least five-fold greater than that required for normal nutritional status appear to be necessary. In addition, selenium must be given continuously for maximum inhibition. Unfortunately, selenium is also known for its profound toxicity, making selenium supplementation a distinct challenge.
Current selenium supplements rely on inorganic forms, such as sodium selenite (Na2SeO3) or sodium selenate (Na2SeO4). While these forms have some value, they are considered more toxic than necessary, and are unlikely to be useful in cancer chemo-prevention. Several organoselenium compounds, which appear to be less toxic in general than the inorganic forms, have been proposed for in vivo use, but the full potential of this strategy has not yet been realized. In general, however, it is very clear that the chemical form in which selenium is introduced consistently shows a marked influence on biological outcomes, including cancer chemoprevention and toxicity.
Selenocysteine is an organic form that is present in the body and is now recognized as the 21st amino acid used in protein synthesis. While it represents a valuable biochemical form, selenocysteine is chemically unstable and difficult to handle, which has undoubtedly deterred its study and use. In addition, even though it possesses greatly reduced inherent toxicity, it still may be too toxic at chemopreventive doses to the therapeutically useful. Accordingly, prodrug forms of selenocysteine that possess reduced inherent toxicity and improved physicochemical properties would be desirable.
Objects of the Invention
It is, therefore, an object of the invention to provide prodrugs and conjugates of thiol- or selenol-containing compounds, such as cysteine, cysteamine, glutathione, selenocysteine, selenocysteamine, and the WR compounds.
Another object of the invention is to provide such thiol- or selenol-containing compounds displaying reduced toxicity and increased clinical utility.
Another object of the invention is to provide such thiol- or selenol-containing compounds with increased lipophilicity that can target a specific organ or region of the body.
Another object of the invention is to provide such thiol- or selenol-containing compounds that can be conjugated to antioxidants, such as vitamin C and E, thus maximizing the effects by providing different agents that work by complementary mechanisms.
Further objects of the invention will become evident in the description below.